I agree with Gunsil.
Lonsdaleite is also known as "hexagonal diamond" and it is an allotrope of Carbon. Allotropes are just the different crystal structures one element can have, like how Ozone and Dioxygen are both forms of Oxygen but are still two completely different materials, so the nickname "hexagonal diamond" is kind of misleading and it isn't a diamond, at least not in the sense we usually think of one.